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houston pushed a commit to branch branch_9_8
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commit 6d3c0c8f169db30f978b0e340f62aaaa590ad514
Author: Houston Putman <[email protected]>
AuthorDate: Fri Feb 14 12:01:34 2025 -0600

    SOLR-17673: Disable multithreaded search at the node level by default 
(#3183)
    
    (cherry picked from commit 970c1fb98ffd31e1bb5ed6d1a33d6baaa96d1627)
---
 solr/CHANGES.txt                                                    | 4 ++++
 solr/core/src/java/org/apache/solr/core/NodeConfig.java             | 3 +--
 .../modules/upgrade-notes/pages/major-changes-in-solr-9.adoc        | 6 ++++++
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/solr/CHANGES.txt b/solr/CHANGES.txt
index 28789e7d138..3da8e752454 100644
--- a/solr/CHANGES.txt
+++ b/solr/CHANGES.txt
@@ -10,6 +10,10 @@ Bug Fixes
 * SOLR-17637: LBHttp2SolrClient can fail to complete async requests in certain 
error scenarios.
   This can cause the HttpShardHandler to indefinitely wait on a completed 
response that will never come. (Houston Putman)
 
+* SOLR-17673: Disable multi-threaded search execution by default at the 
node-level.
+  Users can still opt-in by providing a "indexSearcherExecutorThreads" > 0.
+  (Houston Putman, Varun Thacker, David Smiley, Luke Kot-Zaniewski)
+
 Dependency Upgrades
 ---------------------
 (No changes)
diff --git a/solr/core/src/java/org/apache/solr/core/NodeConfig.java 
b/solr/core/src/java/org/apache/solr/core/NodeConfig.java
index da9fb3a4262..67c71839436 100644
--- a/solr/core/src/java/org/apache/solr/core/NodeConfig.java
+++ b/solr/core/src/java/org/apache/solr/core/NodeConfig.java
@@ -679,8 +679,7 @@ public class NodeConfig {
     // No:of core load threads in cloud mode is set to a default of 8
     public static final int DEFAULT_CORE_LOAD_THREADS_IN_CLOUD = 8;
 
-    public static final int DEFAULT_INDEX_SEARCHER_EXECUTOR_THREADS =
-        Runtime.getRuntime().availableProcessors();
+    public static final int DEFAULT_INDEX_SEARCHER_EXECUTOR_THREADS = 0;
 
     private static final String DEFAULT_CORESLOCATORCLASS =
         "org.apache.solr.core.CorePropertiesLocator";
diff --git 
a/solr/solr-ref-guide/modules/upgrade-notes/pages/major-changes-in-solr-9.adoc 
b/solr/solr-ref-guide/modules/upgrade-notes/pages/major-changes-in-solr-9.adoc
index 834c76fccac..3216e0c04af 100644
--- 
a/solr/solr-ref-guide/modules/upgrade-notes/pages/major-changes-in-solr-9.adoc
+++ 
b/solr/solr-ref-guide/modules/upgrade-notes/pages/major-changes-in-solr-9.adoc
@@ -67,6 +67,12 @@ It is always strongly recommended that you fully reindex 
your documents after a
 In Solr 8, it was possible to add docValues to a schema without re-indexing 
via `UninvertDocValuesMergePolicy`, an advanced/expert utility.
 Due to changes in Lucene 9, that isn't possible any more.
 
+== Solr 9.8.1
+=== Configuration
+In solrconfig.xml, the `indexSearcherExecutorThreads` now defaults to 0.
+Therefore, multi-threaded search execution is disabled at a node-level by 
default.
+It is currently recommended to only use an `indexSearcherExecutorThreads > 0` 
if all query requests sent to Solr opt-in to multiThreaded search.
+
 == Solr 9.8
 === Configuration
 In solrconfig.xml, the `numVersionBuckets` and `versionBucketLockTimeoutMs` 
settings are now obsolete and ignored; a warning will be logged if specified.

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